1. Field of the Invention
The present invention relates to a transmission apparatus and a transmission method employing a layer coding method.
2. Description of the Related Art
Conventionally, a layer coding method has been known as a method for efficient and simultaneous transmission of information sources having different priorities.
The layer coding method is capable of improving transmission efficiency of an information source having a high priority even in an environment with a poor communication state, in the following way. In the layer coding method, a symbol component 1 is assigned a bit sequence which constitutes a high-priority information source, and a symbol component 2 is assigned a bit sequence which constitutes a low-priority information source. Here, the symbol component 1 is assumed to be received with a higher reception quality in a reception apparatus, while the symbol component 2 is assumed to be received with a lower reception quality in the reception apparatus.
With reference to FIGS. 1 to 4, an explanation will be given for a conventional transmission apparatus 20 employing the layer coding method.
The transmission apparatus 20 is configured by applying the layer coding method to the orthogonal frequency division multiplexing (OFDM) method that is one of multi-carrier transmission methods.
As shown in FIG. 1, the transmission apparatus 20 includes a plurality of error correction coding units 211, 212, . . . , a plurality of interleavers 221, 222, . . . , a buffer 23, a layer modulating unit provided with a layer coding unit 24, a multiplexing unit 25 and an OFDM signal generating unit 26.
With reference to FIG. 2, an explanation will be given for operations of the conventional transmission apparatus 20 employing the layer coding method
As shown in FIG. 2, in step S201, each of the error correction coding units 211, 212, . . . , perform error correction coding processing for bit sequences constituting the inputted information sources 1, 2, . . . , respectively.
In step S202, each of the interleavers 221, 222, . . . , perform interleave processing for the bit sequences outputted from the error correction coding units 211, 212, . . . , respectively, after the error correction processing.
In step S203, the buffer 23 stores the bit sequences outputted from the interleavers 221, 222, . . . , respectively, after the interleave processing.
In step S204, the layer coding unit 24 performs layer coding processing for the bit sequences extracted from the buffer 23, according to a priority level of each of the information sources 1, 2, . . . , that constitute the bit sequences.
For example, as shown in FIG. 3, the layer coding unit 24 is configured to perform layer coding processing for a symbol component B1 (1, 1) is assigned a bit sequence which constitutes a high-priority information source 1, and where a symbol component B2 (1, −1) is assigned a bit sequence which constitutes a low-priority information source 2. Here, the symbol component B1 (1, 1) is assumed to be received with a higher reception quality in the reception apparatus and the symbol component B2 (1, −1) is assumed to be received with a lower reception quality in the reception apparatus.
Note that in the example shown in FIG. 3, the modulated signal having performed the layer coding processing is configured of any one of sixteen patterns of symbols with four bits indicated in one symbol.
Additionally, in the example shown in FIG. 3, the symbol component B1 indicates a coordinate point based on the origin point, and the symbol component B2 indicates a coordinate point based on the point A corresponding to the symbol component B1.
Here, in the example shown in FIG. 3, reception quality of the symbol component B1 in the reception apparatus is assumed to be better than reception quality of the symbol component B2 in the reception apparatus. This is because the symbol component B1 indicates only one coordinate point in one quadrant, while the symbol component B2 can potentially indicate four coordinate points in one quadrant.
Moreover, in Japanese Patent Publication No. 2000-31944, disclosed a technique as shown in FIG. 4 for improving transmission efficiency of a BPSK modulated signal, by enabling transmission of an information source 1 even in a poor communication state. Specifically, the transmission is enabled by performing the layer coding unit 24 to assign a bit sequence which constitutes a high-priority information source 1 to a symbol component 1 (I component), and to assign a bit sequence which constitutes a low-priority information source 2 to a symbol component 2 (Q component). Here, the symbol component 1 (I component) is assumed to be received with a higher reception quality in a reception apparatus, while the symbol component 2 (Q component) is assumed to be received with a lower reception quality in the reception apparatus.
Note that, in the example shown in FIG. 4, the modulated signal having performed the layer coding processing is configured of any one of four patterns of symbols with two bits indicated in one symbol.
Here, the reception quality of I component in the reception apparatus is assumed to be better than that of Q component, since the distance 251 between signal points of I component is longer than the distance 252 between signal points of Q component.
In step S205, the multiplexing unit 25 performs, multiplexing processing (scheduling processing) for the modulated signal having performed the layer coding processing, the modulated signal outputted from the layer coding unit 24. In the multiplexing processing, the multiplexing unit 25 assigns the modulated signal to a radio resource divided based on a time and a frequency (an orthogonal signal).
In step S206, the OFDM signal generating unit 26 converts the modulated signal assigned to the radio resource into an OFDM signal, and then transmits the OFDM signal.
However, there has been a problem of deterioration in transmission efficiency by using the above-mentioned conventional transmission apparatus 20. This is because, a bit sequence is not assigned to a part of a symbol component in the modulated signal having performed the layer coding processing, when a length of bit sequence differs between a bit sequence of the information source 1 and a bit sequence of the information sources 2.
Here, consider an example case where the length of the bit sequence which constitutes the information source 1 is longer than the bit sequence which constitutes the information source 2, as shown in FIG. 5A. In this example case, any bit sequence constituting the information source 2 is not assigned to a part of a symbol component 2.
Specifically, if the example shown in FIG. 3 is applied to this example case, the layer coding unit 24 performs the layer coding processing in which bit sequence constituting information source 2 is not assigned to the symbol component B2 and a bit sequence constituting the information source 1 is assigned only to the symbol component B1.
The modulated signal having performed the layer coding processing in this case is configured of any one of four patterns of symbols with two bits indicated in one symbol (that is, the modulated signal in this case has two symbol components).
Meanwhile, if the example shown in FIG. 4 is applied to the aforementioned example case, the layer coding unit 24 performs layer coding processing in which a symbol component 2 (Q component) is not assigned any bit sequence constituting the information source 2, and only a symbol component 1 (I component) is assigned a bit sequence constituting the information source 1.
The modulated signal having performed the layer coding processing in this case is configured of any one of two patterns of symbols with one bit indicated in one symbol.
Moreover, there has been a problem of deterioration in transmission efficiency by using the above-mentioned conventional transmission apparatus 20. This is because, the deterioration is caused, a bit sequence is not assigned to a part of the each symbol component in the modulated signal having performed the layer coding processing, when a transmittable data length of each radio resource is longer than a length of a bit sequence constituting the information sources 1 and 2.
For example, there has been a problem of deterioration in transmission efficiency when the transmittable data length of each radio resource is longer than the length of the each bit sequence constituting the information source 1 and 2 as shown in FIG. 5B. This is because, the deterioration is caused, in such a situation, a part of each the symbol component 1 and 2 in the modulated signal having performed the layer coding processing is not assigned the each bit sequence constituting the information source 1 and 2.